Final Overview

As outlined previously, the myelinated axon is a highly complex structure with impressive dimensions. Linked via specialized interfaces with three different cell types—myeli-nating oligodendrocytes, perinodal astrocytes, and nodal NG2 cells—it displays a remarkable morphological, as well as molecular and functional, heterogeneity along its length. In addition to reflecting a rich set of interactions between the axon and associated glial cells, this polymorphism presents multiple targets for the disease process in multiple sclerosis (MS). In view of the complex structure of the myelinated CNS fiber, and of the more-or-less symbiotic relations between the axon and the glial cells associated with it, it is not surprising that axons are subject to injury in MS, a disease classically considered as primarily affecting myelin. Continued studies of the morphological, molecular, and functional properties of normal myelinated CNS nerve fibers will bring us close to a more complete understanding of the cellular mechanisms that are deranged in MS. It is hoped that this knowledge will enable us to treat and/or prevent the disease in the not-too-distant future.

Figure 23 Longitudinally sectioned myelinated mitral cell dendrites in the adult cat olfactory bulb. (A) Two myelinated dendrites. The top one has four successive thin sheaths labeled 1, 2, and 3+4; the bottom one has two thin sheaths labeled 1 and 2. Arrowheads indicate myelin sheath terminations and asterisks indicate unmyelinated dendritic segments. (x3,500.) (B) Higher magnification of the framed area in (A). Arrowheads indicate myelin sheath ends and arrows point at a 7-|m long stretch of bare dendritic membrane. Note that this dendritic segment (asterisk) is devoid of a nodal differentiation. (x8,160.) (C, D) Details from framed areas in (B) showing lateral loops (asterisks) completely separated from the dendritic membrane (D = dendrite) by glial processes (p). Note that the dendritic membrane lacks specializations at as well as beyond the myelin sheath terminations (arrows). Arrowhead positions correspond to those in (B). (x53,700.) (With permission from Remahl and Hildebrand, 1985.)

Figure 23 Longitudinally sectioned myelinated mitral cell dendrites in the adult cat olfactory bulb. (A) Two myelinated dendrites. The top one has four successive thin sheaths labeled 1, 2, and 3+4; the bottom one has two thin sheaths labeled 1 and 2. Arrowheads indicate myelin sheath terminations and asterisks indicate unmyelinated dendritic segments. (x3,500.) (B) Higher magnification of the framed area in (A). Arrowheads indicate myelin sheath ends and arrows point at a 7-|m long stretch of bare dendritic membrane. Note that this dendritic segment (asterisk) is devoid of a nodal differentiation. (x8,160.) (C, D) Details from framed areas in (B) showing lateral loops (asterisks) completely separated from the dendritic membrane (D = dendrite) by glial processes (p). Note that the dendritic membrane lacks specializations at as well as beyond the myelin sheath terminations (arrows). Arrowhead positions correspond to those in (B). (x53,700.) (With permission from Remahl and Hildebrand, 1985.)

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